JP2002033536A - Optical fiber for optical amplification - Google Patents

Abstract

(57) [Problem] To provide an optical fiber for optical amplification which can amplify a signal light and output it with an ultra-high output, and which has high long-term reliability. SOLUTION: A first cladding layer 2, a second cladding layer 3, and a third cladding layer 4 are provided on an outer peripheral side of a core 1 containing a rare earth metal.
Are provided in this order, and the refractive index is defined as core 1> first cladding layer 2> second cladding layer 3> third cladding layer 4. The second cladding layer 3 is formed of quartz glass, and the third cladding layer 4 is formed of silicone resin. Although the excitation light mainly enters the first cladding layer 2, a part of the excitation light enters the second cladding layer 3, whereby the first cladding layer 2 and the second cladding layer 3 are formed.
To suppress heat generation due to reflection of excitation light at the interface. By making the outer peripheral shape of the XY cross section of the first cladding layer 2 and the second cladding layer 3 non-circular, the excitation light is efficiently reflected, and the signal light propagating through the core 1 is efficiently amplified.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber for amplifying signal light, which is used for, for example, wavelength division multiplexing optical transmission.

[0002]

2. Description of the Related Art With the development of the information society, the amount of communication information tends to increase dramatically. With the increase of such information, wavelength division multiplexing optical transmission (WDM transmission) has been widely accepted in the communication field. The era of wavelength division multiplexing is now approaching. Wavelength division multiplexing optical transmission is an optical transmission method suitable for high-capacity, high-speed communication because light of a plurality of wavelengths can be transmitted by a single optical fiber. Currently, optical amplification optical fibers are applied as optical amplifiers. Wavelength division multiplexing optical transmission is performed at a wavelength that is the gain band of the optical amplifier.

For such wavelength division multiplexing optical transmission,
For example, for the purpose of lengthening the relay interval, it is required to obtain an ultra-high output signal light, and an optical amplification optical fiber that amplifies the signal light and outputs it at an ultra-high output has been proposed. .

FIG. 5 shows a proposal example of this type of optical amplification optical fiber. FIG. 2A is a cross-sectional view of the optical amplification optical fiber taken along an XY section orthogonal to the optical axis Z, and FIG. 2B is an optical amplification optical fiber. Is shown.

[0005] As shown in these figures, the proposed optical fiber for optical amplification comprises a first cladding having a lower refractive index than the core 1 on the outer peripheral side of the core 1 containing a rare earth metal such as erbium (Er). The first cladding layer 2 is provided with a second cladding layer 3 having a lower refractive index than the first cladding layer 2. The first cladding layer 2
Has a square cross-sectional outer peripheral shape (XY cross-sectional outer peripheral shape) when cut along an XY cross section orthogonal to the optical axis Z of the optical fiber. A coating 6 is provided on the outer peripheral side of the second cladding layer 3.

The first cladding layer 2 is made of pure quartz, the second cladding layer 3 is made of silicone resin, and the coating 6 is made of UV resin (ultraviolet curing resin).
And the like.

In the proposed optical fiber for optical amplification, when high-power pumping light is input to the first cladding layer 2, the pumping light propagates through the first cladding layer 2. Here, since the outer peripheral shape of the XY cross section of the first cladding layer 2 is not a perfect circle, among the propagation modes of the excitation light, the helical beam, which is called a helical beam, spirals along the circumference substantially without entering the core. The formation of a propagating propagation mode is suppressed, and the incident excitation light is randomly reflected at the interface between the first cladding layer 2 and the second cladding layer 3 and repeatedly passes through the core 1.

[0008] Therefore, when the outer peripheral shape of the XY section of the first cladding layer 2 is non-circular, the core 1 is smaller than the outer peripheral shape of the XY section of the first cladding layer 2.
Is efficiently amplified by the pumping light, and an ultra-high output light can be obtained. In addition, since the excitation light is incident on the core 1 in a state close to the side incidence by the reflection, the rare earth metal of the core 1 can be sufficiently excited also at this point.

[0009]

By the way, when high-power pumping light is incident on the optical fiber for optical amplification proposed above,
The reflection at the interface between the first cladding layer 2 and the second cladding layer 3 causes scattering and generates heat. Then, there is a problem that the second clad layer 3 is thermally damaged by the heat generation. Therefore, even if the optical fiber for optical amplification proposed above is used for a short time, the thermal damage causes the second clad layer 3 to deteriorate on the input side of the excitation light, resulting in discoloration, peeling, and the like. And the long term reliability was very poor.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned conventional problems, and an object of the present invention is to provide an optical amplifier which can amplify a signal light and output it with an ultra-high output and which has a long-term reliability. An optical fiber is provided.

[0011]

In order to achieve the above-mentioned object, the present invention has the following structure to solve the problem. That is, in the first invention, a first cladding layer having a lower refractive index than the core is provided on the outer peripheral side of the core containing the rare earth metal, and the refractive index is higher than the first cladding layer on the outer peripheral side of the first cladding layer. Is provided, a third cladding layer having a lower refractive index than the second cladding layer is provided on the outer peripheral side of the second cladding layer, and at least one of the first cladding layer and the second cladding layer is This is a means for solving the problem with a configuration in which the cross-sectional outer peripheral shape when cut along the XY cross-section orthogonal to the optical axis Z of the optical fiber is formed as a non-circular shape.

Further, the second invention has a problem that the minimum value of the diameter of the first cladding layer is larger than the mode field diameter of the core in the wavelength band of the signal light, in addition to the structure of the first invention. Is a means to solve.

Further, the third invention has a problem in that, in addition to the structure of the first or second invention, the second clad layer is formed of quartz glass and the third clad layer is formed of resin. Is a means to solve.

In the present invention having the above structure, the first, second, and third cladding layers are sequentially provided on the outer peripheral side of the core, and the core, the first cladding layer, and the second cladding layer are arranged in order of the refractive index. And the third cladding layer, when high-power excitation light is input, the power density of the excitation light of the first cladding layer should be higher than the power density of the excitation light of the second cladding layer. it can. In this case, it is preferable that the second cladding layer is formed of quartz glass.

If the outer peripheral shape of the XY section of the first cladding layer is a non-circular shape, of the propagation modes of the pump light,
The formation of a spiral propagation mode called a helical beam is suppressed, and the incident excitation light is randomly reflected at the interface between the first cladding layer and the second cladding layer and repeatedly passes through the core. Signal light is efficiently amplified by the pump light, and it is possible to obtain an ultra-high output light. Further, since the excitation light is incident on the core in a state close to the side incidence due to the reflection, it is possible to sufficiently excite the rare earth metal of the core also at this point.

In the present invention, a part of the excitation light propagates through the second cladding layer. If the outer peripheral shape of the XY section of the second cladding layer is a non-circular shape, the helical beam is formed in the same manner as described above. The formation is suppressed, and the incident excitation light is randomly reflected at the interface between the second cladding layer and the third cladding layer,
Since the light repeatedly passes through the core while being coupled with the pumping light of the first cladding layer, the signal light incident on the core is more efficiently amplified by the pumping light, and it is possible to obtain ultra-high output light.

In the present invention, the power density of the excitation light of the first cladding layer can be made higher than the power density of the excitation light of the second cladding layer by forming the cladding into a three-layer structure. Most of the excitation light propagates while being reflected at the interface between the first clad layer and the second clad layer. Therefore, compared with a conventional structure having a two-layer clad, a silica clad layer and a resin clad layer are required. It is possible to reduce the optical power that passes while reflecting off the interface with. Therefore, heat generated by scattering at the interface between the second clad layer and the third clad layer can be suppressed, and deterioration of the optical fiber due to the heat can be suppressed.

[0018]

Embodiments of the present invention will be described below with reference to the drawings. In the description of the present embodiment, the same reference numerals are given to the same parts as those in the conventional example, and the overlapping description will be omitted.

FIGS. 1 and 2 show a first embodiment and a second embodiment of an optical fiber for optical amplification according to the present invention, respectively. In these drawings, (a) is a cross-sectional view of the optical fiber for optical amplification of each embodiment when cut along an XY cross section orthogonal to the optical axis Z.
(B) shows the refractive index profile of each optical amplification optical fiber.

As shown in these figures, the optical fiber for optical amplification of the first and second embodiments is an erbium (E)
r) on the outer peripheral side of the core 1 containing a rare earth metal.
A first cladding layer having a lower refractive index is provided; a second cladding layer having a lower refractive index than the first cladding layer is provided on an outer peripheral side of the first cladding layer;
A third cladding layer 4 having a lower refractive index than that of the second cladding layer 3 is provided on the outer peripheral side of the third cladding layer 3. The outer periphery of the third cladding layer 4 is provided with a coating 6, and the coating 6 is formed to have a higher refractive index than the cladding layers 2, 3, and 4.

In the first and second embodiments, both the first clad layer 2 and the second clad layer 3 are cut along the XY cross section orthogonal to the optical axis Z of the optical fiber. The shape (the outer peripheral shape in the XY cross section) is made to be a square shape which is a non-circular shape. Note that, for example, one side (one side of the above square shape) of the outer periphery of the cross section of the first cladding layer 2 is 60
μm, and one side of the outer periphery of the cross section of the second cladding layer 3 is 180 μm.
μm. The diameter of the core 1 was 8.5 μm, the diameter (outer diameter) of the third cladding layer 4 was 350 μm, and the diameter (outer diameter) of the coating 6 was 400 μm.

Further, in the first and second embodiments,
The diameter of the first cladding layer 2 is made larger than the mode field diameter of the core 1 (even at the smallest diameter).

In the first embodiment, the core 1
The first cladding layer 2 is made of quartz glass, and is made of quartz glass doped with r, Yb, P, Al, and Ge. The second cladding layer 3 is made of F-doped quartz glass, and the third cladding layer 4 is made of silicone resin. Coating 6
Is made of UV resin.

In the first embodiment, the relative refractive index difference Δ1 of the core 1 with respect to pure quartz is 0.4%, the relative refractive index difference Δ3 of the second cladding layer 3 with respect to pure quartz is -0.7%,
The relative refractive index difference Δ4 of the third cladding layer 4 with respect to pure quartz is −
3.0%.

In this specification, each relative refractive index difference Δ
1 to Δ4, the relative refractive index of the core 1 when the vacuum refractive index is 1 is n1, the relative refractive index of the first cladding layer 2 is n2, the relative refractive index of the second cladding layer 3 is n3, The relative refractive index of the cladding layer 4 is defined as n4, and the relative refractive index of pure quartz is defined as nc, and is defined by the following equations (1) to (4). The unit is%.

Δ1 = [{(n1) ² − (nc) ² } / 2 (n1) ² ] × 100 (1)

Δ2 = [{(n2) ² − (nc) ² } / 2 (n2) ² ] × 100 (2)

Δ3 = [{(n3) ² − (nc) ² } / 2 (n3) ² ] × 100 (3)

Δ4 = [{(n4) ² − (nc) ² } / 2 (n4) ² ] × 100 (4)

In the second embodiment, the second clad layer 3, the third clad layer 4, and the coating 6 are all the first clad layer.
In the second embodiment, the first cladding layer 2 is formed of Ge-doped quartz glass, and the relative refractive index of the first cladding layer 2 with respect to pure quartz is different from that of the second embodiment. The difference Δ2 is 0.6%. Also,
The difference between the relative refractive index difference Δ1 of the core 1 and the relative refractive index difference Δ2 of the first cladding layer 2 is 0.4% as in the first embodiment.
In order to achieve the above, the core 1 is made of silica-based glass so that the relative refractive index difference Δ1 of the core 1 with respect to pure quartz becomes 1.0%.
It is formed by doping r, Yb, P, Al, and Ge, respectively.

The first and second embodiments are configured as described above. The optical amplification optical fiber of each embodiment has a three-layer structure having first, second and third cladding layers. Since the second cladding layer 3 is formed of silica-based glass, a part of high-power excitation light is input to the first cladding layer 2 in each embodiment, and the remaining excitation light is The portion can be input to the second cladding layer 3. In addition,
The ratio of the excitation light input to the first cladding layer 2 is made larger than the ratio of the excitation light input to the second cladding layer 3.

Then, the input pumping light propagates through the first cladding layer 2 and the second cladding layer 3, but the first and second
In the embodiment, since the outer peripheral shapes of the first and second cladding layers 2 and 3 in the XY section are both square,
The formation of the helical beam among the propagation modes of the excitation light is suppressed. Therefore, the excitation light incident on the first cladding layer 2 is randomly reflected at the interface between the first cladding layer 2 and the second cladding layer 3, repeatedly passes through the core 1, and
The excitation light incident on the second cladding layer 3 is
At the interface between the first cladding layer 4 and the third cladding layer 4,
The signal light incident on the core 1 is efficiently amplified by the pumping light, and an ultra-high output light can be obtained.

Further, since the excitation light is incident on the core in a state close to the side incidence by the reflection, it is possible to sufficiently excite the rare earth metal of the core also in this respect.

In the first and second embodiments, the cladding has a three-layer structure, and the excitation light is distributed to and incident on the first cladding layer 2 and the second cladding layer 3. Since a part of the light propagates while being reflected at the interface between the second clad layer 3 and the third clad layer 4, the first clad layer 2 and the second clad layer 2 are compared with a conventional structure having a clad having a two-layer structure. The optical power that passes while reflecting the interface with the two cladding layers 3 can be reduced. Therefore, according to the first and second embodiments, heat generated due to scattering due to reflection at the interface between the first clad layer 2 and the second clad layer 3 can be suppressed, and deterioration of the optical fiber due to the heat can be suppressed. .

The inventor actually performed a high-temperature and high-humidity test using the experimental apparatus shown in FIG. 3 to obtain the output optical power output from the optical amplification optical fibers of the first and second embodiments. A comparison was made with the conventional optical amplification optical fiber shown in FIG. 5 as a comparative example.

The diameter of the core 1 and the outer diameter of the coating 6 in the optical fiber for amplification of the comparative example are the same as those of the above embodiments, and one side of the square of the first cladding layer 2 is approximately 18
0 μm, the outer diameter of the second cladding layer 3 is 350 μm, and the difference between the relative refractive index difference Δ1 of the core 1 and the relative refractive index difference Δ2 of the first cladding layer 2 is set to 0 in the same manner as in the above embodiments. .
4%.

As shown in FIG. 3, in the high-temperature and high-humidity test, the center of an optical amplification optical fiber (indicated by reference numeral 8 in FIG. 3) was wound around a SUS bobbin 10 having a diameter of 600 mm. The pumping light power output from each optical amplification optical fiber was obtained after passing the elapsed time (H; time) shown in Table 1 under an atmosphere of 85 ° C. and 95 RH%.

[0038]

[Table 1]

The measurement of the output pumping light power is based on the output 2 W
The excitation light from the five 980 mm laser diodes 12 is spatially coupled at the portion shown in FIG. A in a state where the multimode optical fibers 15 connected to each laser diode 12 are bundled, and the collimator lens 13 is Then, the light enters the optical fiber for output amplification through the optical fiber for output detection, and the output light from the optical fiber for optical amplification is received and detected by the light receiver 14 for detection.

As a result, the results shown in Table 1 were obtained, and the optical fiber for optical amplification of the first and second embodiments showed an excitation light output of about 6 W before the test even after the lapse of 2000 hours in the high-temperature and high-humidity test.
(Incident light rate of 60%) could be maintained.

The optical fiber for optical amplification of the comparative example is
After the high-temperature and high-humidity test, discoloration due to heat generation at the interface between the first cladding layer 2 and the second cladding layer 3 and peeling of the silicone resin forming the second cladding layer 3 are observed, and the silicone resin partially disappears. However, deterioration of the optical fiber for optical amplification of each of the above embodiments was not observed at all, and from this result and Table 1, it was confirmed that the long term reliability of each of the above embodiments was high. Was.

Further, according to each of the above embodiments, the minimum value of the diameter of the first cladding layer 2 is made larger than the mode field diameter of the core 1, so that the signal light is directed to the outer peripheral side of the first cladding layer 2. The signal light can be suppressed from protruding and propagated, and the signal light can be appropriately amplified and propagated.

It should be noted that the present inventors have found that, in each of the above embodiments, when the pump light is distributed and incident on the first cladding layer 2 and the second cladding layer 3, the amplification characteristic of the signal light due to the pump light is deteriorated. As shown in FIG. 4, in order to confirm that no signal light source 20 is provided on the input side of the optical amplifying optical fiber (indicated by reference numeral 8 in FIG. 4), five 980 mm light sources having an output of 2 W are provided on the output side. By connecting the laser diodes 12 respectively,
The signal light was pumped backward, the signal light was received by the light receiver 14, and the signal light output was measured. Also in this figure, reference numeral 13 indicates a collimator lens, and reference numeral 17 indicates a dichroic mirror.

As a result, the signal light output of the first embodiment and the signal light output of the comparative example are 1.6 W, and the signal light output of the second embodiment is 1.5 W, which are almost the same value. It was confirmed that the amplification characteristics were almost the same.

It should be noted that the present invention is not limited to the above-described embodiment, but can adopt various embodiments. For example, in each of the above embodiments, the third cladding layer 4 is formed of a silicone resin, but the third cladding layer 4 may be formed of another resin such as a fluorine-based resin.

In each of the above embodiments, the outer peripheral shape of the first cladding layer 2 and the second cladding layer 3 in the XY section are both square, but if at least one of these shapes is a non-circular shape, Good. The non-perfect circular shape may be a polygonal shape such as a square as in the above embodiments, an elliptical shape, a shape in which a notch is formed in a circle, or a true shape. An appropriate shape other than a circle can be set.

[0047]

According to the present invention, since the first, second, and third cladding layers are sequentially provided on the outer peripheral side of the core, for example, a part of high-power excitation light is input to the first cladding layer. The rest of the pump light can be input to the second cladding layer. The amplification efficiency can be improved when the ratio of the excitation light input to the first cladding layer is made larger than the ratio of the excitation light input to the second cladding layer, and the second cladding layer is formed of quartz glass.

According to the present invention, since the outer peripheral shape of at least one of the first cladding layer and the second cladding layer is a non-circular shape, the outer peripheral shape of the XY cross-section is a non-circular shape. The pump light that is incident on the core is suppressed from forming a spiral propagation mode that does not enter the core among the propagation modes, and the incident pump light efficiently passes through the core repeatedly, so that the signal light that enters the core Can be efficiently amplified by the excitation light, and an ultra-high output light can be obtained.

According to the present invention, as described above, the cladding has a three-layer structure, and the excitation light is applied to the first cladding layer and the second cladding layer.
Since the light can be distributed to the cladding layer and incident, a part of the excitation light can be propagated while being reflected at the interface between the second cladding layer and the third cladding layer. Compared with the configuration having the above, it is possible to reduce the optical power that passes while reflecting at the interface between the first cladding layer and the second cladding layer. Thus, according to the present invention,
Heat generated by scattering due to reflection at the interface between the first clad layer and the second clad layer can be suppressed, deterioration of the optical fiber due to the generated heat can be suppressed, and long-term reliability can be improved.

Further, according to the present invention in which the diameter of the first cladding layer is larger than the mode field diameter of the core, it is possible to suppress the signal light from protruding to the outer peripheral side from the first cladding layer and to propagate the signal light appropriately. And can be propagated.

Further, according to the present invention in which the second clad layer is formed of quartz glass and the third clad layer is formed of resin, the second clad layer can be made to easily transmit the excitation light. The effect of confining the excitation light inside the interface between the second clad layer and the third clad layer by increasing the relative refractive index difference between the second clad layer and the third clad layer can be very effectively exerted. .

[Brief description of the drawings]

FIG. 1 is a main part configuration diagram showing a first embodiment of an optical fiber for optical amplification according to the present invention.

FIG. 2 is a main part configuration diagram showing a second embodiment of the optical fiber for optical amplification according to the present invention.

FIG. 3 is an explanatory diagram showing a system configuration for performing a high-temperature and high-humidity test on an optical fiber for optical amplification.

FIG. 4 is an explanatory diagram showing a system configuration for confirming optical amplification characteristics of an optical amplification optical fiber.

FIG. 5 is an explanatory diagram showing an example of a conventionally proposed optical amplification optical fiber.

[Explanation of symbols]

 Reference Signs List 1 core 2 first cladding layer 3 second cladding layer 4 third cladding layer 6 coating

Claims (3)

[Claims] A first cladding layer having a lower refractive index than the core; and a second cladding layer having a lower refractive index than the first cladding layer on the outer peripheral side of the first cladding layer. Providing a cladding layer, providing a third cladding layer having a lower refractive index than the second cladding layer on the outer peripheral side of the second cladding layer,
At least one of the first cladding layer and the second cladding layer has a noncircular cross-sectional outer shape when cut along an XY cross-section orthogonal to the optical axis Z of the optical fiber. Optical fiber. 2. The optical fiber according to claim 1, wherein the minimum value of the diameter of the first cladding layer is larger than the mode field diameter of the core in the wavelength band of the signal light. 3. The optical fiber for optical amplification according to claim 1, wherein the second cladding layer is formed of quartz glass, and the third cladding layer is formed of resin.